In rail rapid transit service, a large percentage of the available cars are placed in "lay-over" status or service between peak operating periods, during which time the compressed air supply system normally remains activated to provide compressed air primarily for a parking brake. Due to normal leakage associated with such compressed air supply systems, the air compressor periodically operates to maintain the system air pressure within the operating range required during normal operation, such as 130-150 psi, for example. Accordingly, compressor operation occurs on a relatively heavy duty cycle which results in excessive compressor and motor wear.